This change swaps the CommonJS require() syntax in the Webpacker
application.js pack template file and in documentation examples with ES
module import syntax.

Benefits of this change include:

Provides continuity with the larger frontend community: Arguably, one of
the main draws in adopting Webpacker is its integration with Babel to
support ES module syntax. For a fresh Rails install with Webpacker, the
application.js file will be the first impression most Rails developers
have with webpack and Webpacker.  Most of the recent documentation and
examples they will find online for using other libraries will be based
on ES module syntax.

Reduces confusion: Developers commonly add ES imports to their
application.js pack, typically by following online examples, which means
mixing require() and import statements in a single file. This leads to
confusion and unnecessary friction about differences between require()
and import.

Embraces browser-friendliness: The ES module syntax forward-looking and
is meant to be supported in browsers. On the other hand, require()
syntax is synchronous by design and not browser-supported as CommonJS
originally was adopted in Node.js for server-side JavaScript. That
webpack supports require() syntax is merely a convenience.

Encourages best practices regarding optimization: webpack can statically
analyze ES modules and "tree-shake", i.e., strip out unused exports from
the final build (given certain conditions are met, including
`sideEffects: false` designation in package.json).